As technical developments and demands on mobile devices are increasing, demands on secondary batteries as an energy source is being rapidly increasing. Among the secondary batteries, lithium secondary batteries having high energy density and voltage, long cycle life, and low discharge rate are commercialized and widely used.
However, the lithium secondary battery has a limitation that the life thereof decreases rapidly via repeating charge and discharge. Particularly, the limitation is more serious at high temperatures. The reason is that an electrolyte may be decomposed due to water in the battery or other factors, an active material may be deteriorated, or the internal resistance of the battery may increase.
A positive electrode active material for a lithium secondary battery, which is being actively researched, developed and used, is LiCoO2 with a layered structure. LiCoO2 may be easily synthesized and has good electrochemical properties including life property, and is the most widely used material. However LiCoO2 has low structural stability, and the application thereof to a battery with high capacity is limited.
As the substituents of the positive electrode active material, various lithium transition metal oxides such as LiNiO2, LiMnO2, LiMn2O4, or LiFePO4 have been developed. LiNiO2 has merits of providing the battery properties of high discharge capacity, however is hardly synthesized by a simple solid phase reaction and has low thermal stability and cycle property. In addition, lithium manganese oxides such as LiMnO2 or LiMn2O4 have merits of good thermal stability and low cost, however have limitations of a small capacity and inferior properties at high temperatures. Particularly, for LiMn2O4, some products are commercialized at low cost; however the life property thereof is not good due to Jahn-Teller distortion owing to Mn3+. Since LiFePO4 is inexpensive and safe, a lot of research is being conducted for the use in a hybrid electric vehicle (HEV), however the application thereof to another fields is hard due to low conductivity.
Due to such circumstances, a lithium nickel manganese cobalt oxide, Li(NixCoyMnz)O2 (where x, y, and z are atomic partial ratios of independent oxide composite elements and satisfy 0<x≦1, 0<y≦1, 0<z≦1, and 0<x+y+z≦1), receives much attention as the substituting positive electrode active material of LiCoO2. This material is cheaper than LiCoO2 and has merits of being used under a high capacity and a high voltage. However, the material has demerits of not providing good rate characteristic and life property at high temperatures. In order to increase the structural stability of the lithium nickel manganese cobalt oxide, the amount of Li relative to the amount of a transition metal included in the oxide is increased.
Recently, as the size of portable devices such as mobile phones and tablet computers is gradually miniaturized, batteries applied thereto are also required to be miniaturized together with high capacity and energy. In order to increase the energy per unit volume of a battery, the packing density of an active material or a voltage is required to be increased. In order to increase the packing density, active materials having a large size are preferable. However, the active materials having a large size have a relatively small surface area, and thus, an active area making contact with an electrolyte may be also narrow. The narrow active area may be kinetically unfavorable, and relatively low rate characteristic and initial capacity may be attained.